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BB10005 COPY > Protein trafficking 3 > Flashcards

Protein trafficking 3 Flashcards

1
Q

How can proteins be selectively recruited into buds?

7) Some proteins are passively included in the budding vesicle. Some of these proteins may need to be returned to the ER.

A
  1. For example some integral membrane proteins have cytoplasmic domains that will interact with coat proteins.
  2. This will concentrate them into buds.
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2
Q

How can lumenal cargo proteins be selectively recruited into buds?

A
  1. They can interact with cargo receptors that span the ER membrane and interact with coat proteins.
  2. The cargo and coat are indirectly linked via protein-protein interactions.
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3
Q

What proteins may be excluded from entering budding vesicles?

A
  1. Those that mis-fold in the ER.
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4
Q

How does the ER act as a quality control station in the secretory pathway?

A
  1. Only allows fully folded proteins, proteins which have reached appropriate stage of biosynthesis to leave ER e.g. antibodies
  2. Two heavy and two light chains and need to assembled to be allowed to leave
  3. BiP- binds to unfolded proteins
  4. Retained in ER- So protein BiP is bound to is also retained in ER
  5. BiP remains bound until the fully assembled protein so now the protein can leave the ER
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5
Q

What is the function of BiP in antibody formation?

A

It keeps the immunoglobin from entering the ER exit when it is not fully assembled.

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6
Q

What is retrograde transport?

A
  1. When proteins are returned to the ER after originally being transported from the ER to the golgi.
  2. Some proteins are passively included in the budding vesicle. Some of these proteins may need to be returned to the ER.
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7
Q

Where is a KDEL sequence found?

A
  1. Often found in ER resident proteins

2. At C-termini.

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8
Q

What makes up KDEL?

A

Lysine, Aspartic acid, glutamic acid and leucine.

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9
Q

How can proteins undergo retrograde transport?

10) COP1 coat binds to dileucine motifs but not at ER- ARF coordinate its assembly
11) COP2- doesn’t bind to golgi as not SAR1-GEF

A
  1. KDEL containing proteins are recognised by KDEL receptors (transmembrane proteins)
  2. and are selectively packaged into COPI coated vesicles for retrograde traffick back to the ER.
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10
Q

How can the KDEL receptor be reused?

A
  1. When in the ER the KDEL receptor releases the KDEL containing protein
  2. Returns to the Golgi (empty) in COPII vesicles
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11
Q

How does a vesicle know the correct membrane to fuse with?

A
  1. Rab’s and SNARE proteins give vesicles and target membranes a molecular identity
  2. help direct transport vesicles to their target membrane- specific recognition event
  3. Fusion can only occur if the molecular identities of the two membranes are compatible- Docking event- recognition between target membrane and vesicle
  4. Only compatible membranes will stably interact and fuse.
  5. Each snares on ER are only compatible with snares on the golgi (not with plasma membrane)
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12
Q

What is the difference between tSNAREs and vSNAREs?

A
  1. tSNAREs = target SNARE proteins

2. vSNARE = vesicle SNARE proteins.

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13
Q

What is the specific way in which the two SNARE proteins interact?

A
  1. SNARE pairing brings two membranes close together and ‘forces’ them to fuse. – SNAREs coil together
  2. As the two membranes are brought close together water is ‘squeezed out’.
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14
Q

What is a fusion pore?

A
  1. Fusion of two bilayers forms a fusion pore

2. A channel through which secretions are released from the vesicle to the cell exterior.

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15
Q

What are the intermediates before the formation of a fusion pore?

A

Stalk and hemifusion.

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16
Q

How does botox work?

A
  1. It cleaves SNARE proteins so that neurotransmitter vesicles cannot fuse with the plasma membrane.
  2. This causes paralysis.
17
Q

What is an example of protein modification?

A
  1. Glycosylation.

2. One modification is the addition of sugars onto proteins.

18
Q

Where does N-linked glycosylation occur?

A

In the ER.

19
Q

Where do O-linked modifications occur?

A
  1. In the lumen of golgi apparatus

2. Occurs on serine or threonine amino acids

20
Q

What transfers the oligosaccharide from the lipid to the protein?

A

Oligosaccharyl transferase.

21
Q

How does N-linked glycosylation occur?

A
  1. Sequence of 3 amino acids NXS/T
  2. Recognised by oligosaccharide transferase which adds a sugar on
  3. Sugars are transferred from dolichol (a lipid) onto proteins containing NXS/T sequences in the lumen of the ER.
22
Q

What does the golgi do?

A
  1. Further modification of oligosaccharides and protein.
  2. Processing enzymes are differentially distributed in the stack.
  3. Early acting enzymes in cis cisternae, late acting in trans-cisternae
  4. Trans- cargo is sorted into vesicles to go to different places
23
Q

What is the difference between the cis and trans golgi stacks?

A
  1. Cis face receive vesicles from the ER
  2. Trans face package vesicles for delivery to the plasma membrane.
  3. The Cis-Trans route goes through the medial golgi
24
Q

What is the difference between constitutive secretion and regulated secretion?

A
  1. Regulated secretion involves a receptor recognising a signal such as hormones/neurotransmitter
  2. Transduce signal into cell
  3. Recognised by mechanism that allows vesicle to fuse
  4. E.g insulin by high glucose levels
25
Q

Give some examples of acid hydrolases found in lysosomes.

A
  1. Nucleases
  2. Proteases
  3. Glycosidases
  4. Lipases
  5. Phosphatase
  6. Sulfatases
  7. Phsopholipases
26
Q

How are proteins transported to lysosomes?

A
  1. Special trafficking pathways for delivering proteins to lysosomes
  2. Diverted sideways ( not plasma membrane) and go through endosomes and then get to lysosome
  3. pH in ER is quite high- 7
  4. pH in lysosome is approx. 5- due to proton pump which pumps protons into lysosome - hydrolyses ATP to ADP and Pi.
27
Q

What coordinates budding from ER

A
  1. Sar1 coordinates budding from ER because Sar1-GED is located in ER membrane
  2. When Sar1 is loaded with GTP there is a conformation change and there is a partial exertion of the alpha helix into the membrane
  3. Starts to recruit inner coat proteins
  4. One of the units bind to activated cell 1, the other unit binds to cytoplasmic tails of proteins present in ER
  5. Recruits certain proteins into the bud which is forming
  6. Diacidic motif is recognised by sec 23 and sec 24
  7. Specificity between protein in ER and coat complex as Proteins which don’t contain these type of signals are not selectively recruited into the bud forming
  8. The outer coat proteins then start to recruit
  9. Luminal proteins can also be recruited by interactions with these proteins
28
Q

What happens to remove the vesicle from the ER

A
  1. A scission event- removal of vesicle from ER membrane
  2. The vesicles must then be targeted to the next membrane (e.g golgi apparatus)
  3. Vesicle with a protein coat can fuse with the next membrane if the protein coat is removed
29
Q

How is the protein coat removed

A
  1. GTP hydrolysis in sar1
  2. Sar 1 loses affinity for helix and for sec 23 and sec24 subunits so they dissociate
  3. Left with naked vesicle
30
Q

Which proteins are not glycosylated and why

A
  1. Cytoplasmic proteins do not get glycosylated even if they have an NXS/T sequence in them as oligosaccharide transferase is not there.
  2. Only proteins which enter the lumen of ER or Golgi are.
  3. Glycolipids are only found in lumen, not cytoplasmic side
31
Q

Which proteins are not glycosylated and why

A
  1. Cytoplasmic proteins do not get glycosylated even if they have an NXS/T sequence in them as oligosaccharide transferase is not there.
  2. Only proteins which enter the lumen of ER or Golgi are.
  3. Glycolipids are only found in lumen, not cytoplasmic side
32
Q

Where does disulphide bond formation happen

A
  1. In ER as oxidising

2. Not Cytoplasm as reducing environment

33
Q

Describe how low density lipoproteins are transported to lysosomes

A
  1. LDL receptor is trafficked to the cell surface from the ER.
  2. The receptor recycles between the plasma membrane and endosomes picking up LDL (low density lipoprotein) at the PM and releasing it in endosomes (acidic pH).
  3. LDL is trafficked to lysosomes.
  4. The receptor returns to the PM.

BB10005 COPY (23 decks)

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